Math.random() in tests

Let's assume following source code generating random number with Math.random() and faker.random.number():

var faker = require('faker');

faker.seed(1);

console.log(
  "Faker goes:", 
  faker.random.number({ max: 100 })
);

console.log(
  "Math.random() goes:", 
  Math.random() * 100
);

And now let's try to run it several times in a row:

❯ node index.js
Faker goes: 42
Math.random() goes: 24.270154608840078

❯ node index.js
Faker goes: 42
Math.random() goes: 17.379030134115037

❯ node index.js
Faker goes: 42
Math.random() goes: 66.8433058100395

One quite common thing to do while writing tests is to write down what data got generated during the test run and assert against that value:

let myRandomInt = faker.random.number({ max: 100 });
// let myRandomInt = Math.random() * 100;

assert.equals(myRandomInt, 42, 'Coincidence? I think not');

Now while there are better methods to write tests, this is a quick win and when done right can work quite well. As you can see from the values generated above when you use Math.random() you will get different result every time. While when using faker the results seem stable.

Faker will always give you the same results when all the calls to it are exactly the same till your call. The problem arises when you for some reason add another call to faker before your call:

faker.random.number(); // Extra faker call

let myRandomInt = faker.random.number({ max: 100 });
assert.equals( // This will fail ☹️
  myRandomInt, 
  42, 
  'Coincidence? I think not'
);

How to solve this? Using seed, which will reset the pseudo-random sequence:

faker.random.number(); // Extra faker call

faker.seed(1);
let myRandomInt = faker.random.number({ max: 100 });
assert.equals( // It works again ✨
  myRandomInt, 
  42, 
  'Coincidence? I think not'
);

Conclusion

If you want to hard-code assert expected values in your test, you should make sure that you:

1. In your tests use stable random generator like faker instead Math.random().
2. Pin faker.seed(x) to a constant value before generating data for each test.

Photo by Riho Kroll on Unsplash